The present invention relates to a method for obtaining control signals to aim an antenna toward a signal source, in which a sum and difference signal are derived from the signal received by the antenna, the difference of phase between sum and difference signal is subject to a cyclic phase change, and these signals are superposed so that a signal is produced from which control values are derived for the movement of the antenna into the desired direction.
In satellite transmissions or in radar systems, for example, reflector antennas are equipped with servo follow-up systems for tracking targets. For this purpose, systems operating according to the so-called monopulse method, as described, e.g. in German OS No. 1,936,692, are in wide use. In this method, criteria for the deviation of the antenna boresight axis from the desired direction are derived from at least two different radiation patterns, or beams. The normal antenna beam whose field intensity maximum lies on the boresight axis of the antenna serves as the reference beam. The additionally generated radiation beam or beams--called difference beams--have a point of zero field intensity on the boresight axis and a phase reversal.
With the aid of the reference beam, which is also called the sum beam, and with the use of the difference beams, it is possible to generate deviation proportional signals by means of synchronous demodulation. Such signals can be utilized to point the antenna back onto its target.
The sum and difference beams are generated either from the radiation beams of various waveguide modes excited in the antenna feeder system--this is then called the multimode monopulse method--by way of sum or difference formation of the received signals from a plurality of individual feedhorn. The system described by Notthof, "Apollo antenna fastens on the beam to the moon," in Electronics, May 1, 1967, pages 80-86, is based on the latter method. Here, two difference signals are derived by individual feedhorns from the received signals, and these two difference signals are oriented in the directions of the cartesian coordinates x and y, respectively. The deviation information is determined in that these two difference signals are alternatingly superposed on the sum signal, with each difference signal being subjected to a cyclic phase reversal between the two phase states 0.degree. and 180.degree..
If the transmitted signals are linearly polarized signals two difference signals are required, as described above, to derive the deviation information.
With circular polarization, however, a single difference signal oriented in polar coordinates is sufficient for this purpose. Such a difference signal oriented in polar coordinates can be derived from a circular field distribution (e.g. the H.sub.01 mode) in the antenna feeder system or from the radial field distribution (e.g. the E.sub.01 mode). It must also be mentioned that, even with linear polarization, a difference signal can be obtained which is oriented in polar coordinates. Such signal is obtained by vector addition from the two difference signals oriented in cartesian coordinates x and y.